Field of the Invention
The present invention relates to a semiconductor device and a semiconductor device manufacturing method.
Description of Related Art
A semiconductor device having a stacked structure formed by stacking a plurality of semiconductor substrates (silicon substrates) is known. In such a semiconductor device, silicon of one surface of the stacked silicon substrates is etched to decrease a thickness thereof. An example of the semiconductor device having such a structure is a backside illumination (BSI) solid-state imaging device.
In the BSI solid-state imaging device, light is incident on a surface (a back surface) opposite to a surface (a front surface) on which circuit elements such as photoelectric conversion units converting incident light in a photoelectric conversion manner are formed. Accordingly, in the BSI solid-state imaging device, a lot of light is made to be incident on the photoelectric conversion units formed on the front surface of the silicon substrate by etching extra silicon on the back surface of the silicon substrate to make silicon substrate to be thinned.
However, when the silicon substrate is simply thinned, the thinned silicon substrate is wound in a roll shape or cracked. Accordingly, in a semiconductor device which is thinned by etching silicon on one surface of stacked silicon substrates such as the BSI solid-state imaging device, a configuration in which a silicon substrate to be thinned is held (supported) by another silicon substrate is employed.
For example, in the BSI solid-state imaging device, by stacking a silicon substrate (hereinafter referred to as a “second silicon substrate”) other than a silicon substrate (hereinafter referred to as a “first silicon substrate”) in which circuit elements such as photoelectric conversion units are formed, the first silicon substrate to be thinned is held (supported) by the second silicon substrate. At this time, in the BSI solid-state imaging device, only the first silicon substrate is thinned by stacking the second silicon substrate on a front surface of the first silicon substrate, and then etching extra silicon on the back surface of the first silicon substrate.
In recent semiconductor manufacturing processes, several etching methods for removing extra silicon of one silicon substrate in a semiconductor device having a configuration in which a plurality of silicon substrates are stacked and thinning the silicon substrate have been put into practice. A wet etching method has been widely used as the etching method of removing extra silicon of one silicon substrate in a semiconductor device having a configuration in which a plurality of silicon substrates are stacked and thinning the silicon substrate.
The wet etching method is a method of etching silicon using an alkaline etching solution capable of performing anisotropic etching, such as a nitrohydrofluoric acid solution, a potassium hydroxide (chemical formula: KOH) solution, or a tetramethylammonium hydroxide (TMAH) solution, as described in Japanese Unexamined Patent Application, First Publication No. 2013-062382. In the wet etching method, silicon is etched by immersing wafer substrates, in which a plurality of semiconductor devices to be thinned are formed, in an alkaline etching solution in the stacked state. Accordingly, overall silicon exposed from the stacked wafer substrates is etched. Therefore, in the wet etching method, silicon which should not be removed by etching, for example, a second silicon substrate holding (supporting) a first silicon substrate to be thinned in the above-mentioned BSI solid-state imaging device, needs to be protected so as not to be exposed to the etching solution. As a method of protecting silicon which should not be removed by etching, a method of forming an oxide film by heat treatment or the like as a protective film for protecting the silicon substrate or the like is used. This is because the alkaline etching solution exhibits high selectivity to an oxide film, that is, does not etch the oxide film, and thus silicon on a side having the oxide film formed thereon is not etched even when the oxide film is exposed to the etching solution.
As the first silicon substrate to be thinned, a wafer substrate having a silicon-on-insulator (SOI) structure in which a device layer and a silicon support substrate layer are bonded by a buried oxide (BOX) layer to form a single wafer substrate is used. That is, the first silicon substrate includes three layers of a device layer, a BOX layer, and a silicon support substrate layer before being thinned. In a wafer substrate having an SOI structure, transistors, wires, and the like, which are circuit elements of a semiconductor device, are formed on the device layer side and the silicon support substrate layer serves as extra silicon to be removed by etching. For example, in the above-mentioned BSI solid-state imaging device, circuit elements such as photoelectric conversion units are formed in the device layer, and the silicon support substrate layer is removed by etching. Accordingly, when etching is performed using the wet etching method, the silicon support substrate layer is etched by stacking the second silicon substrate on a surface of the device layer side of the first silicon substrate and then immersing the stacked body in an alkaline etching solution. At this time, the circuit elements of the semiconductor device (for example, photoelectric conversion units in the BSI solid-state imaging device) formed in the device layer are protected by a BOX layer as an oxide film similar to the method of protecting silicon which should not be removed by the above-mentioned oxide film. In this way, by using the wafer substrate having the SOI structure as the first silicon substrate to be thinned, only the silicon support substrate layer of the wafer substrated of the SOI structure can be etched using the wet etching method.
In the wet etching method, an oxide film is formed as a protective film for protecting silicon which should not be removed by etching. The oxide film is formed using the wet etching method before etching is performed in semiconductor manufacturing processes, that is, in processes of manufacturing a wafer substrate in which a plurality of semiconductor devices are formed. Accordingly, in an etching process using a general wet etching method, the oxide film is not substantially etched by an alkaline etching solution exhibiting high selectivity to the oxide film, that is, having a high selection ratio to the oxide film.
However, in the second silicon substrate stacked on the first silicon substrate, an uneven state of the oxide film which is formed before the etching process occurs at a place at which a silicon composition has a defect or a place including a foreign substance. That is, a defect is generated in the oxide film formed on the second silicon substrate. Accordingly, at a place at which a defect is unintentionally generated in the oxide film, the selection ratio may decrease and the etching speed (etching rate) by an etching solution may increase in comparison with a place at which no defect is generated in the oxide film.
In the process of manufacturing a wafer substrate using the semiconductor manufacturing processes, a scratch may be formed on the surface of the oxide film formed on the second silicon substrate. At a place at which the oxide film is scratched, the etching rate may also increase in comparison with a place at which the oxide film is not scratched.